1. Field of the Invention
The present invention relates to a light emitting device having a vertical structure, and more particularly to a light emitting device and corresponding manufacturing method having a vertical structure for enhancing a light escape efficiency.
2. Discussion of the Related Art
Light emitting diodes (LEDs) are semiconductor light emitting devices, which convert current into emitted light. For example, a red LED using a GaAsP compound semiconductor has been available since 1962 and is used together with a GaP:N-based green LED as a image display light source in electronic apparatuses.
In addition, the wavelength of light emitted from such an LED depends on the semiconductor material used to fabricate the LED. That is, the wavelength of the emitted light depends on the band gap of the semiconductor material representing an energy difference between the valence-band electrons and the conduction-band electrons.
One example of a semiconductor compound is Gallium nitride (GaN). Further, the GaN compound has been used in LEDs because it is possible to fabricate a semiconductor layer capable of emitting green, blue or white light using the GaN compound in combination with other elements, for example, indium (In), aluminum (Al), etc.
Thus, using GaN in combination with other appropriate elements, it is possible to adjust the wavelength of light to be emitted. Accordingly, when GaN is used, it is possible to appropriately determine the materials of a desired LED in accordance with the characteristics of the apparatus to which the LED is applied. For example, it is possible to fabricate a blue LED useful for optical recording or a white LED for replacing a glow lamp.
On the other hand, initially-developed green LEDs were fabricated using GaP. However, because GaP is an indirect transition material that causes a degradation in efficiency, the green LEDs fabricated using this material cannot produce pure green light. Instead, an InGaN thin film is used to fabricate a high-luminescent green LED.
Also, because of the advantages of GaN-based LEDs, the GaN-based LED market has rapidly grown. In addition, GaN-based LEDs have been developed to exhibit light emission efficiency superior to that of glow lamps. Currently, the efficiency of GaN-based LEDs is substantially equal to that of fluorescent lamps. Thus, it is expected that the GaN-based LED market will grow significantly.
However, despite the rapid advancement in technologies of GaN-based semiconductor devices, the fabrication of GaN-based devices includes high-production costs. This disadvantage is closely related to difficulties associated with growing a GaN thin film (epitaxial layer) and subsequently cutting of the finished GaN-based devices.
In more detail, such a GaN-based device is generally fabricated on a sapphire (Al2O3) substrate. This is because a sapphire wafer is commercially available in a size suited for mass production of GaN-based devices, supports a GaN epitaxial growth with a relatively high quality, and exhibits a high processability in a wide range of temperatures. Further, sapphire is chemically and thermally stable, and has a high-melting point enabling implementation of a high-temperature manufacturing process.
Also, sapphire has a high bonding energy (122.4 Kcal/mole) and a high dielectric constant. In terms of a chemical structure, the sapphire is a crystalline aluminum oxide (Al2O3). Meanwhile, because sapphire is an insulating material, available LED devices manufactured using a sapphire substrate (or other insulating substrate) are limited to a lateral or vertical structure.
In the lateral structure, all metal contacts used of injecting electric current into LEDs are positioned on the top surface of the device structure (or on the same substrate surface). On the other hand, in the vertical structure, one metal contact is positioned on the top surface, and the other contact is positioned on the bottom surface of the device after the sapphire (insulating) substrate has been removed.
In addition, a flip chip bonding method has been widely used. In the flip chip bonding method, an inverted LED chip, which has been separately prepared, is attached to a sub-mount of, for example, a silicon wafer or ceramic substrate having an excellent thermal conductivity. However, the lateral structure LED or the LED made using the flip chip method suffers from problems associated with poor heat release efficiency, because the sapphire substrate has a heat conductivity of about 27 W/mK, thus leading to a very high heat resistance. Furthermore, the flip chip method requires several photolithography process steps, thus resulting in complicated manufacturing processes.
Therefore, LED devices having a vertical structure are being used because the sapphire substrate is removed in vertical structure LEDs. In more detail, a laser lift off (LLO) method is used to remove the sapphire substrate. For example, as shown in FIG. 1, a GaN thin film including an n type GaN layer 2, an active layer 3, and a p type GaN layer 4 are formed over a sapphire substrate 1. A p type electrode 5 is also formed over the GaN thin film.
The LLO method is then applied to the chip as fabricated in the above-mentioned manner to completely remove the sapphire substrate 1. In addition, in the LLO method, stress is applied to the GaN thin film upon incidence of a laser beam. Therefore, to separate the sapphire substrate 1 and GaN thin film from each other, a laser beam having a high energy density is used. The laser beam then decomposes the GaN into a metal element, namely, Ga, and a nitrogen gas (N2).
Then, after the substrate 1 is removed, an n type electrode 7 is formed on the exposed n type GaN layer 2, as shown in FIG. 2, to fabricate the vertical LED structure. In addition, as shown in FIG. 2, the n type GaN layer 2 is arranged at the uppermost portion of the chip structure. Therefore, the area of a contact region of the n type GaN layer 2 considerably affects the total light emission efficiency.
In addition, although it is advantageous in terms of light escape to reduce the area of the contact region, there may be a problem such as an increase in the operating voltage, because the reduced contact region causes an increase in the total resistance of the device or insufficient current spreading. Further, because the GaN material has a refractive index of 2.35, the angle through which light from the LED is externally emitted without being fully reflected within the LED is limited to 25° from a vertical line when the GaN material is directly in contact with air having a refractive index of 1.